The lithium ion secondary battery, a kind of electrochemical elements, is being considered applicable to portable devices, automobiles, electric tools, electric chairs, and electricity storage systems both for family use and business use, in view of its characteristics of a high energy density. In particular, in portable devices, it is widely used as a power source of cell phones, smartphones or tablet type PCs.
As applicable apparatus of the lithium ion secondary battery being spread, it has been demanded to increase its capacity as well as to improve its batteries' properties in various aspects. In particular, as a secondary battery, its improvement of the charge discharge cycle characteristic is strongly demanded.
Usually, in a negative electrode active material of the lithium ion secondary battery, a carbon material such as graphite which is capable of insertion and desorption of lithium (Li) ions is widely used. On the other hand, Si or Sn or a material including such an element have been examined as a material capable of insertion and desorption of more amounts of lithium (Li) ions, and a compound SiOx having a structure that fine particles of Si are dispersed in SiO2 is particularly focused on. Also, since these materials have a low conductivity, it has been proposed to make them into a structure in which the surface of the particles is coated with a conductive material such as carbon (Patent References No. 1 and No. 2).
In the conventional SiOx as disclosed in Patent References No. 1 and No. 2, a change in volume can be caused by the charge and the discharge, and therefore, the battery properties tend to show a remarkable drop as repeating charge discharge cycles. Then, another approach was proposed by making a negative electrode composition layer have a low density (1.4 g/cm3 or less), and a negative electrode active material use an artificial graphite of 15 μm or more and 20 μm or less, a pitch coat graphite of 10 μm or less, and a carbon-coated SiO (1-10% by weight with respect to the whole weight of all the negative electrode active material), in order to improve the load characteristics and the cycle caused characteristic with respect to a large amount of electric currents of the battery (Patent Reference No. 3).